This invention relates to the field of telemetry systems for transmitting information from the bottom of a well hole to the surface. More particularly, this invention relates to the field of mud pulse telemetry where information detected down the well is transmitted to the surface by pressure pulses created in a circulating mud stream in the drill string.
The desirability and effectiveness of well logging systems where information is sensed in a well hole and transmitted to the surface through mud pulse telemetry has long been recognized. Systems of this type, i.e. mud pulse telemetry systems, provide the driller at the surface with a means for quickly determining various kinds of information down the well, most particularly information about the location and direction of the drill string at the bottom of the well.
Because of the tremendous investment already made in drill pipe and drill collars, it is highly desirable that a bore hole telemetry system be compatible with existing drilling equipment and require minimum or no modification to the drill pipe and drill collars. Mud pulse telemetry is well known to offer an effective solution since it does not rely upon conductor wires to the surface or other mechanisms which may necessitate modification to existing hardware and provides a very fast communication link to the surface since the pulses travel at the speed of sound through the mud. In mud pulse telemetry systems, the telemeter is usually in the form of a valve which intermittently restricts the flow of mud within the drill string, and the valve is usually located in the vicinity of the drill bit. The telemeter may be lowered on a wire line located within the drill collar, but it is more usually formed as an integral part of a special drill collar inserted into the drill string near the drill bit. Representative disclosures of the prior art in mud pulse telemetry systems may be found in U.S. Pat. Nos. 2,677,790, 2,901,685, 2,973,505, 2,964,116, 3,309,656, 3,065,416, 3,693,428, 3,737,843, 3,764,970, 3,764,969, 3,764,968, and 3,770,006, the reference herein to such prior art patents being merely for purposes of illustration and not a complete listing of all prior art in this field.
A continuous column of mud is circulated within the drill string from the surface of the well to the drill bit at the bottom of the well during normal drilling operation. The basic operational concept of mud pulse telemetry is to intermittently restrict the flow of mud as it passes through a down hole telemeter valve to thereby create pressure pulses in the mud stream which travel to the surface at the speed of sound through the drilling mud. The information sensed down the well and which is to be transmitted to the surface is used to intermittently actuate the valve which restricts the mud flow, thereby transmitting pulses or digital information, and the pulses are detected at the surface and transformed into electrical or other signals which can be decoded and processed to reveal the transmitted information.
In typical oil and gas well drilling, mud is circulated through the interior of the drill pipe at flow rates ranging from 300 to 1000 gallons per minute. The mud pulse telemeter must operate to partially restrict this flow, and therefore must control a larger amount of energy. The telemeter valve must actuate quickly to create a pressure pulse, and the intermittent flow restriction must be sufficient to create a pressure rise which will, after attenuation from travelling through the mud to the top of the well, be detectable at the surface. At these typically high flow rates of the mud, considerable force and work are required to actuate the telemeter valve in the manner necessary to create the desired pressure pulses.
A downhole telemeter which is capable of forcefully driving the telemeter valve up into the mud stream must contain a power source sufficiently large to perform the required work. A typical power source discussed in a literature consists of a turbine driven by the mud flow to power an electric generator or other device to actuate the pulse valve. This approach, i.e. of the mud turbine, requires a large energy source, and presents design complications from the need to package the entire telemeter system within the rather narrow diameter of the drill string so that it may be compatible with existing drilling equipment. A telemetry system which is capable of performing the desired functions with a smaller amount of control energy is extremely desirable. Such a system can lend itself to size reduction or even miniaturization and can be easily packaged within the confines of existing drill pipes and drill collars. Furthermore, if input power requirements can be made low enough, power sources other than mud driven turbines, such as high temperature batteries, can be used.